Front Page Titles (by Subject) CHAPTER XXIII.: THE RELATIONS OF LIKENESS AND UNLIKENESS. - The Principles of Psychology
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CHAPTER XXIII.: THE RELATIONS OF LIKENESS AND UNLIKENESS. - Herbert Spencer, The Principles of Psychology 
The Principles of Psychology (London: Longman, Brown, Green and Longmans, 1855).
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THE RELATIONS OF LIKENESS AND UNLIKENESS.
§ 94. At length continued analysis has brought us down to the relations underlying not only all preceding relations, but all processes of thought whatever. From the most complex and most abstract inferences of the developed man, down to the most rudimentary intuitions of the infant, all intelligence proceeds by the establishment of relations of likeness and unlikeness. Duly to realize this fact, we must glance at the successive conclusions arrived at in preceding chapters.
In the most perfect kinds of compound quantitative reasoning, we found that each of the several intuitions through which any conclusion is reached, not only involves the relation of likeness under its highest form—that of equality—but involves it in the most various ways. We found that in descending step by step to the lower kinds of reasoning, the intuitions of likeness included in each ratiocinative act, become less numerous and less perfect; but that to the last, likeness of relations is necessarily involved. The classification of objects, we found to imply a perception of the likeness of a new group of relations to a before-known group, joined with more or less unlikeness of the individual attributes; while recognition implies exact likeness, both of the individual attributes and their relations, to those of groups before known. And we further saw that the perception of a special object is impossible save by thinking of it as like some before-known class or individual. The perception of Body, as presenting its three yorders of attributes, we found to imply a classing of the several attributes, their relations to each other, and the conditions under which they are disclosed, with like attributes, relations, and conditions. It was shown that our ideas of Space, Time, and Motion, arise by a discovery of the equivalence of certain states of consciousness, serial and simultaneous; and further, that no particular space, time, or motion can be thought of, without the relation of likeness being involved. More recently, we have seen that the higher orders of relations are severally resolvable into relations of likeness and unlikeness whose terms have certain specialities and complexities. Similarity, was defined as the cointension of two connatural relations between states of consciousness which are themselves like in kind but commonly unlike in degree. Cointension, we found to be, likeness in degree between either changes in consciousness that are like in kind, or states of consciousness that are like in kind. It was shown that coextension is the likeness of two composite states of consciousness, in respect of the number and order of the elementary relations of coexistence which they severally include. Coexistence, was resolved into two sequences whose terms are exactly alike in kind and degree, exactly unlike, or opposite, in their order of succession, and exactly alike in the feeling which accompanies that succession. Connature was defined as likeness in kind between either two changes in consciousness, or two states of consciousness. And each of these relations we found to have its negative, in which unlikeness is the thing predicated.
Seeing thus, that the knowing of successive states and changes of consciousness as like or unlike, is that in which thinking essentially consists, we have next to inquire what is the essential nature of those phenomena in consciousness which we signify by the words likeness and unlikeness. Are the relations of likeness and unlikeness definable? And if so, what are they?
§ 95. Things cannot be truly defined except in terms more general than themselves: and hence, unless there is some relation underlying the relations of likeness and unlikeness, they must be indefinable. Strictly speaking, no such more general relation exists. The only relation yet remaining to be dealt with, is one that is co-ordinate with them—one that lies upon the same plane with them—one that is in fact another side of the same mental phenomena. All that is possible for us, is, to describe likeness and unlikeness in terms of this remaining relation; and to describe this remaining relation, when we come to it, in terms of likeness and unlikeness—to exhibit them as the necessary complements of each other.
This premised, the question above asked will be most readily answered by comparing the relations of likeness and unlikeness together. The essential nature of each will best be shown by contrast with the other. In what then consist the difference between the two mental processes by which these relations are disclosed?
If I cut in two a sheet of coloured paper—say blue—and place the pieces at some distance apart; and if I also place at some distance apart, two other pieces which are of different colours—say red and green; I have in the first pair a relation of likeness, and in the second pair a relation of unlikeness. In what consists the knowledge of each of these relations? On glancing from one of the blue pieces to the other, I am conscious of passing from one state to another state, which is new in so far as it is separate from, and subsequent to, the first, but which is not new in any other respect. On glancing from the red to the green, I am conscious of passing from one state to another state, which is new not only as being subsequent, but which is otherwise new. Suppose now that I place the blue pieces quite close together, joining the two edges that were cut; and that I also place the red and green pieces close together. What happens? The two blue pieces are not now known in two distinct states of consciousness: the two states of consciousness practically merge into one. The red and green pieces however, placed no matter how close, still produce two states when contemplated. Similarly again with odours. A flower when smelt at, produces a certain continuous state of consciousness. If another flower of the same kind be joined with it, and the two are moved about under the nostrils, the successive scents may be made to seem as continuous as the scent of one. But if the flowers are of different kinds, they will, when successively smelt at, produce different states of consciousness. The like is true of sounds. A sustained note from a wind or stringed instrument, may be perfectly homogeneous, or it may be interrupted by some scarcely appreciable flaw, serving nominally to divide it into two notes that are exactly alike. But while, when we listen to such a note, consciousness may with almost equal propriety be considered in one state or two states; when we listen to any musical interval, we very decidedly experience two states. And this antithesis between the relations of likeness and unlikeness, will be yet further elucidated, when it is remarked that not only do the states of consciousness which we call like, lapse insensibly into one state, but that any one state of consciousness having an appreciable continuity, may be conceived as divided out into a series of like states.
From all which it will be sufficiently manifest, that by the words unlike and like, we signify the occurrence or non-occurrence of change in consciousness. Leaving out of sight for a moment that fleeting state of consciousness which marks a transfer of the attention, and which strictly considered is a change, we may say that by unlikeness and likeness we mean respectively, change and no change in consciousness. The two terms of a relation of unlikeness, are two states of consciousness forming the antecedent and consequent of a change in consciousness: the two terms of a relation of likeness, are the antecedent and consequent of what, in one sense, is no change; seeing that it leaves consciousness in the same condition as before.
As implied however, this is but an approximate statement—an adumbration, which, if interpreted strictly, describes an impossibility. For, as the relation of likeness implies two terms, two states of consciousness; and as two states of consciousness, if not themselves different, cannot exist as separate states unless they are divided from each other by some state that is different; it follows that a relation of likeness implies a change, or rather changes, in consciousness. Accurately speaking, therefore, a relation of likeness consists of two relations of unlikeness which neutralize each other. It is a change from some state A to another state B (which represents the feeling we have while passing from one of the like things to the other), and a change from the state B to a second state A; which second state A would be indistinguishable from the first state were it not divided from it by the state B, and which merges into such first state when the state B disappears, from the approximation of the two like stimuli in space or time.
Very many relations of unlikeness similarly consist of two relations of unlikeness, which, however, do not neutralize each other. In all cases where the two terms of the relation do not follow through consciousness in juxtaposition—as when the unlike things looked at are some distance apart, or when between unlike sounds or odours a brief interval of time elapses—there are three states of consciousness involved; the original state A, the transition state B, and that state of which we predicate unlikeness, C. But the primordial relation of unlikeness is one consisting of two states only. When two notes differing in pitch, strike the ear in rapid succession, so as to leave no time for any intervening thought or sensation—when a flash of lightning for a moment dispels the darkness—when any one state of consciousness is supplanted by another state, there is established a relation of unlikeness.
Thus, then, the relation of unlikeness is the primordial one—is the relation involved in every other relation; and can itself be described in no other way than as a change in consciousness.